With the rapid market expansion in the fields of laptop computers and cellular phones, there has been a growing demand for small-sized, high-energy-density and high-capacity secondary cells for use in such machines or instruments. To meet such demand, secondary cells which utilize an electrochemical reaction associated with a charge transfer, with alkali metal ions such as lithium ions serving as charge carriers, have been developed. In particular, lithium ion secondary cells are utilized as high-capacity secondary cells high in energy density and excellent in stability in various electronic machines and instruments. In such lithium ion secondary cells, a lithium-containing transition metal oxide is generally used as an active material in a positive electrode, and carbon as an active material in a negative electrode, and charging and discharging are carried out by utilizing insertion and elimination reactions of lithium ions into and from these active materials.
In recent years, a secondary cell in which a radical compound is utilized as an electrode active material directly contributing to an electrode reaction have been proposed for further capacity increases (see Patent Document 1).
In the secondary cell, as a solvent of an electrolyte in which charge carriers move between the negative electrode and the positive electrode, organic solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, tetrahydrofuran, dioxofuran, sulfolane, dimethylformamide, dimethylacetoamide, and N-methyl-2-pyrrolidone are used. Each of these organic solvents can be used alone, or two or more of these can be used in combination. Furthermore, as the radical compound, polymers having a stable radical in their side chain such as poly(2,2,6,6-tetramethyl-4-piperidinoxy methacrylate), poly(2,2,5,5-tetramethyl-2-pyrrolidinoxy methacrylate), and poly(2,2,5,5-tetramethyl-2-pyrrolinoxy methacrylate) are used.
In accordance with these conventionally-proposed radical compounds, however, performance stability of the secondary cells is insufficient because stability to a solvent is insufficient and the electrode active material is dissolved by the solvent of the electrolyte.
Thus, the inventors of the present invention made researches for solving the above problems and found a method for producing a radical compound excellent in stability to a solvent. As a result, the inventors filed the patent application relating to a method for producing a crosslinked poly(meth)acrylic acid nitroxide compound, comprising the step of polymerizing a predetermined (meth)acrylic acid compound in the presence of a crosslinking agent and a method for producing a secondary-cell electrode using the compound (see Patent Document 2).    Patent Document 1: Japanese Kokai Publication 2002-304996    Patent Document 2: WO2005/116092